1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device, and more particularly to a nitride semiconductor light-emitting device having an AlN polycrystalline or amorphous substrate and a process for producing the same.
2. Description of the Related Art
Generally, a great deal of attention has been directed to nitride semiconductor crystals as a material for use in preparing visible and ultraviolet light LEDs and blue-green color optical devices such as light-emitting diodes or laser diodes, because they produce light with a broad spectrum including the entire visible light region and also the ultraviolet light region.
In order to prepare a high efficiency optical device with such nitride semiconductor crystals, there is essentially required a technique which enables a nitride semiconductor to grow into a high quality single crystal thin film. However, the III-A group nitride semiconductors are not suitable for general substrates due to their lattice constant and thermal expansion coefficient and thus have a difficulty in growing a single crystal thin film.
To grow nitride semiconductor crystals, a sapphire (Al2O3) or SiC substrate is limitedly employed only. For example, nitride semiconductor crystals may be grown on the sapphire substrate by heteroepitaxy methods using Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), and the like.
But, despite use of such a nitride semiconductor single crystal substrate, it is difficult to directly grow a high quality nitride semiconductor single crystal on the substrate due to inconsistency of a lattice constant and a thermal expansion coefficient therebetween, and thus a low temperature nucleus-growth layer and a buffer layer are additionally used. FIG. 1 is a cross-sectional side view of a conventional nitride semiconductor light-emitting device.
As shown in FIG. 1, a conventional nitride semiconductor light-emitting device; which is designated by reference number 10, includes an n-type nitride semiconductor layer 15 formed on a sapphire substrate 11, an active layer 16 having a multi-well structure 16 and a p-type nitride semiconductor layer 17. An n-electrode 19a was formed on a region of the exposed portion of the n-type nitride semiconductor layer 15 by removing and exposing some portions of the p-type nitride semiconductor layer 17 and the active layer 10. A transparent electrode 18 containing Ni and Au, and a p-electrode 19b were formed on the p-type GaN semiconductor layer 17.
Further, a buffer layer was formed on the sapphire substrate in order to grow high quality nitride semiconductor crystals. As the buffer layer, a low temperature nucleus-growth layer such as AlxGa1-xN wherein x is between 0 and 1, is usually used.
However, even when a nitride semiconductor single crystal was grown on the sapphire substrate after forming the low temperature nucleus-growth layer, the nitride semiconductor single crystal had crystal defects of about 109 to about 1010 cm−2. In particular, these crystal defects propagate in the vertical direction and thus exhibit adverse effects responsible for leakage of electrical current.
On the other hand, the conventional sapphire or SiC substrate may have disadvantages of high cost, lower thermal conductivity, and lower mechanical properties, resulting in increased production costs and deterioration of device characteristics, as compared to an AlN polycrystalline or amorphous substrate. But, the AlN polycrystalline or amorphous substrate is not suitable for growth of the nitride single crystal layer and thus is not usually used as a substrate for the nitride semiconductor light-emitting device.